The invention relates to an apparatus and a method for adjusting a collimator, especially a collimator for an X-ray testing machine having an x-ray source and elements for generating a primary beam.
Alignment and adjustment of a collimator and detector apparatus in an X-ray machine has a decisive impact on selectivity and, consequently, a decisive impact on material recognition and the probability of detecting an object. This is especially true in X-ray machines that employ X-ray diffraction, where an exact adjustment is critical for precisely measuring and determining the detected material based on the exact angular position of the collimator and detector apparatus relative to the X-ray beam.
German published Patent Application DE 195 10 168 A1 discloses an example of adjusting a collimator and detector apparatus in an X-ray testing machine. In this case, an automatic readjustment is performed prior to each measurement. The collimator and detector apparatus, which comprises a plurality of collimators with detectors respectively located behind them, is disposed on a carrier unit. The carrier unit includes an additional, central collimator that is oriented toward the focus of an X-ray-generating X-ray source, or is oriented with each readjustment. During an adjustment, the central beam (primary beam) emitted by the X-ray source and passing through the central collimator in an exact orientation, generates detection signals on adjacent individual detectors disposed behind the central collimator, with the signals being of identical magnitude when the adjustment is exact.
It is an object of the invention to provide an apparatus and a method for the adjustment of a collimator in an X-ray testing machine that is structurally simpler than those previously known, and is completely automatic.
The above object generally is achieved according to a first aspect of the invention by an apparatus that includes a collimator for an X-ray testing machine having an X-ray source for generating X-ray radiation and elements for generating a primary beam, and wherein the collimator has a central, blind-bore-like opening and first and second detection devices, which are spatially separated disposed and spaced one behind the other inside the central opening, for orienting the collimator relative to the primary beam.
The above object generally is achieved according to a second aspect of the invention by a method for adjusting a collimator in an X-ray testing machine relative to a primary X-ray beam by using first and second X-ray detection devices, which are spatially separated, disposed and spaced one behind the other along the primary beam, to spatially orient the collimator so that the primary beam impacts the centers of the first and second detection devices at a right angle, the method comprising, in the first step, moving the collimator until a signal at the first detection device is maximal; in a second step, rotating the collimator in two independent planes about a point preferably located near the center of the first detection device until a signal from the second detection device is maximal. This is achieved by first rotating the collimator in plane one. An intensity maximum on the second detection device is obtained and subsequently rotating the collimator in plane two, which is independent of plane one until maximum is achieved on both detection devices.
The concept underlying the invention is to perform the adjustment with the aid of a detection system disposed in the collimator and comprising at least first and second spatially separated detection devices that are placed one behind the other. In the case of a homogeneous attenuation of a primary beam emitted by an X-ray source through the first detection device of the detection system, the remaining beam intensity is used once more for the final orientation of the collimator with the aid of the second detection device. In this way, it is possible to attain an exact, coaxial orientation of the collimator on or along the axis of the primary beam, thereby assuring a higher measuring precision within an X-ray machine. The further apart the first and second detection devices are spaced, the more precise the orientation of the collimator inside an X-ray machine will be.
For the spatially local orientation of the collimator in a first point, the first detection device is moved in the primary beam until a signal generated in the process is maximal, that is, the most intense. For an optimum orientation of the collimator along the primary beam, the orientation is then effected relative to a further point in the collimator. This is attained with the aid of the second detection device, with the collimator being rotated in a (higher) plane about an imaginary point (located as close as possible to the center of the first detection device) in two independent planes until the signal is also maximal in the second detection device, without minimizing the signal at the first detection device. After the maximum in the first rotational plane has been established in this manner, a further rotation takes place in the second rotational plane; here, too, the center of rotation lies as close as possible to the center of the first detection device. The optimum orientation is assured if the intensity maximum is established in both detection devices.
In a simple embodiment, a collimator arrangement comprising, for example, apertured diaphragms, can be mounted in front of each of the two detection devices in the collimator. These apertures or collimator adapt the respective detection surface to the beam diameter of the primary beam. When using detection devices whose sensitive surfaces coincide sufficiently with the beam diameter, the collimator arrangement can be omitted.
A semiconductor, gas or scintillation counter that is sufficiently thin and homogeneous can be used as the first detection device. Such a counter only effects a slight attenuation of the primary beam, and essentially retains the intensity distribution of the incident beam in the transmitted beam. A semiconductor, gas or scintillation counter can also be used as the second detection device. Its absorption properties must, however, be matched to thexe2x80x94on average, higherxe2x80x94energy of the quanta of the resulted beam transmitted through the first detection device.
The two spatially separated detection devices can each respectively comprise a four-quadrant detector, at least two individual detectors, a detector array or position-sensitive detector having multiple-segmented diodes.
The method is preferably used to adjust round-slot collimators with a detector, e.g. crystal detector, located behind them. In this case, the system is set at a predetermined angle relative to the primary beam for an exact measurement employing X-ray diffraction.
It is also possible to adjust a simple collimator/detection device that is used, for example, in conventional material detection. In this case, the first detection device is embodied as a detector for the relatively lower-energy component of the radiation, and the second detection device is embodied as a detector for the relatively higher-energy component.
The invention is described in detail below by way of an embodiment illustrated in the drawings.